This invention relates to a method which makes it possible to ascertain the flow condition of a liquid metal comprehensively from a moving direction or the like of a peak point of a cross-correlation function of fluctuation signals occurring in the outputs of an electromagnetic sensors disposed in the flowing liquid metal. Though not restricted, in particular, the present invention relates to a method which is especially suited for characteristic measurement (flow velocity measurement) of a core of aliquid metal cooled nuclear reactor, core anomaly detection and diagnosis of anomaly cause (void detection, detector self-calibration/diagnosis), and the like, by way of example.
An eddy current flow sensor, for example, has heretofore been known as an instrument for electromagnetically measuring the flow velocity of an electrically conductive fluid.
Such a flow sensor employs, as illustrated in FIG. 1, a detector 7 which consists of an excitation coil 6 and two detection coils 4, 5 disposed on both sides of the excitation coil 6 (that is, a 3-coil type), and the detector 7 is placed in the flowing direction of a conductive fluid. A so-called 5-coil type has also been known in which excitation coils are further disposed outside the detection coils. In any case, an a.c. magnetic field generated by the excitation coil 6 is such that the magnetic flux .phi. is distributed symmetrically with respect to the axis when the fluid F is static (curve 1). When the fluid F moves in the direction indicated by an arrow, however, the magnetic flux .phi. is deformed on the downstream side (curve 2). Accordingly, a voltage S.sub.1 induced in the detection coil 4 on the upstream side decreases with an increase in the flow velocity but increases with a decrease in the flow velocity, whereas a voltage S.sub.2 induced in the detection coil 5 on the downstream side exhibits the reverse behavior with respect to the voltage S.sub.1. The difference between S.sub.2 and S.sub.1 provides a signal depending upon the flow velocity, i.e. a flow velocity signal e.sub.F. This is the principal of measurement in the conventional flow sensor.
However, "fluctuation" exists in the output S.sub.1 and S.sub.2 of the detection coils 4 and 5, respectively, and this fluctuation pulsates with respect to the time averaged signal. By determining the cross-correlation function R.sub.1,2 (.tau.) of fluctuations of S.sub.1 and S.sub.2, one of the inventors of the present invention found that the flow velocity could be obtained by dividing the effective distance L between both detection coils by a delay time .tau..sub.o at the peak point of R.sub.1,2 (.tau.), on the basis of the concept that the delay time .tau..sub.o is inversely proportional to the flow velocity. As a result, one of the inventors of the invention proposed a novel flow sensor (Japanese Patent Laid-Open No. 128363/1978). In accordance with this system, the flow velocity can be obtained irrespective of the magnitude of the signal itself and hence, it can be applied to self-calibration of the flow sensor. Since only the flow velocity is taken into consideration, however, no method has yet been found which makes it possible to comprehensively ascertain the flow condition including the state of voids.
There has conventionally been known a void detection method which detects the voids using the magnitude of an r.m.s. (root mean square) value of fluctuation of the flow velocity signal. However, this method involves the drawbacks that disturbance, which has no relation to the voids, cannot be eliminated and consequently, the level of background noise is likely to become greater, thereby lowering the void detection sensitivity.
On the contrary, in the field of liquid metal cooled nuclear reactors, for example, development of such a method has been eagerly required which makes it possible to accurately ascertain the flow condition of a liquid metal by associating flow velocity detection with void detection, for monitoring the flow velocity in a fuel subassembly, for detecting core anomaly or for diagnosing the cause of such anomaly, and moreover, which method enables self-diagnosis of the detector.